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Friday, August 29, 2008

Emergent Gravity

I am on my way back from the Emergent Gravity conference at MIT, contemplating what I've heard and learned. The aim of this meeting was to bring together condensed matter physicists with those tireless seekers looking for a fundamental theory unifying classical gravity with quantum field theory. Should such a theory exist then the features we observe might only be collective variables, emerging from a more basic underlying structure. Much like the properties of liquids are eventually a consequence of the dynamics of its molecules, the spacetime we live in might only arise in a macroscopic limit from a more fundamental theory. One would expect then both areas, condensed matter and quantum gravity, to share common approaches when going from a microscopic to a macroscopic description, and there to possibly be similar features like phase transitions or modifications of symmetries in the small distance limit.

This past week we have heard about emergence of gravitons on a quantum bosonic model (Zheng-Cheng Gu, arXiv:gr-qc/0606100v1), the emergence of diffeomorphism (Jorge Pullin, arXiv:gr-qc/0606121v1) and the emergence of spacetime and matter in Group Field Theory (Daniele Oriti, arXiv:0710.3276v1), to only mention a few. My head is still spinning, and I don't really know where to place all this information, so please don't ask for details. Fotini gave an update on Quantum Graphity - A condensed matter model of emergent geometry (arXiv:0801.0861v2). I wasn't very convinced when I first heard of that model, but they've made some refinements to the approach and I see a real chance that maybe some day I indeed manage to make sense of it. Lee talked about his paper with Joao about the possible observational consequences of a phase transition in the early universe from a non-geometrical to a geometrical phase (arXiv:astro-ph/0611695v3), and my talk this morning was just a summary of my Minimal Length model that you of course know all about so I won't bother you with the details.

A highlight was certainly Stephen Wolfram's talk on Tuesday evening in which he introduced his idea that the universe and everything we know and like fundamentally arise from a cellular automaton. He advertised his book "A New Kind of Science", in which, allegedly, the emergence of the universe and all theories we usually deal with is explained. Stephen's claims are certainly bold. By choosing the right updating rules for the automaton, so he says, he gets not only a manifold with Lorentzian symmetry, but also Einstein's Field equations. Unfortunately, I couldn't quite follow his argumentation - the talk was a bit confused (to put it mildly) and about 1 hour over time, but it was certainly interesting. I think I will need to have a look at this book before I come to any conclusions.

Thursday afternoon, we had a panel discussion with Jorge Pullin, Max Tegmark, Xiao-Gang Wen, and Bei-Lok Hu, moderated by Olaf Dreyer. The topic circled around the question what are aspects of emergence, why it is so hard to quantize gravity, and whether this is the right path to follow at all. The issue of background indepence was briefly touched, so was the question of observable predictions and whether or not time is fundamental. It was an interesting exchange, though with a certain lack of disagreement.

I had prepared a 5 min bonus to my talk because I thought I might finish earlier, but then I finished remarkably in time and didn't need it. It was to mention some of my thoughts on the merits of emergence and our quest for a fundamental theory on a very general level, probably quite bloggable. So I thought, you'd get the bonus instead - if I come around to writing it up that is.

To brag a bit: we were staying a the Kendall Hotel in Cambridge, right next to MIT. A very nice place that I can warmly recommend.

50 comments:

I would definitely like to hear further commentary from you (& other physicists) about ANKOS. I think L. Motl (when he was at Harvard) used to hang out with SW, & likes him. It would be interesting to hear Lumo's view on CA. S. Wolfram's claim that CA/Cellular Automata is the TOE. I know a couple of Caltech profs (1 is nuclear physicist by training, other is Math PhD) who know SW. Chris Adami dabbles in Digital Life (teaches a Caltech course, & wrote a book), & is one of SW's most outspoken critics:

SW came to Caltech in early '05, & there was a panel session with him, J. Preskill, C. Adami. Unfortunately, I missed it. JP was not optimistic that CA would be a revolution, & of course neither was C. Adami. SW's response was, this was a typical "status-quo" response to a revolutionary theory. I know a Caltech researcher (scholar in DNA Computing) who dislikes SW immensely. Because his friend (Caltech postdoc) was sued by SW/Wolfram Research over confidentiality issues in some research done at WR.

There were 3 "baseline" review of ANKOS by the physics community: Scott Aronson (then at Berkeley CS dept, now at MIT/EE Dept), S. Weinberg, Leo Kadanoff/U. of Chicago. Everyone else was an emotional response, to SW's pomposity in claiming he invented much of CA. I took great interest in this controversy, since my Dad was on the search committee that brought SW to UIUC (triple appt: Math, Physics, Computer Science) from Princeton. My dad really wanted to work with him, on his specialty CFD/Computational Fluid Dynamics. SW had some early papers @UIUC on the subject. There was some conflict, objections to his company & a supposed "conflict of interest". He was asked to leave. That led to Wolfram Research & development of Mathematica..& the rest is history. 1 of the co-founders is Theodore Gray (plucked out of Berkeley grad school/Chemistry), who is an alumni of my high-school (Class of '82). I'm Class of '75. Another co-founder was Dan Grayson (UIUC math dept), whose son is a recent Caltech Physics PhD. He is with a mobile robotics company (started by his MIT undergrad friends) in Las Vegas, he's into Robotics. My Jumplive.com "collided" with the "robotic offroad race", the DARPA Grand Challenge, with Google founders & S. Wozniak in attendance. Caltech had a team, but they sucked bad: Stanford won in a Red Bull sponsored Volkswagen Touareg. This was my PhD field, "Artificial Intelligence/Computer Vision/Robotics".

I think CA is part of the continuum set of models for TOE:

1) Mathematical ModelMax Tegmark's theory

2) Computer ModelCA/Cellular Automata (SW's pet theory)

3) Physical Model[ insert here, I need to rely on your expertise ]

I don't think there is a single TOE, it's a family of models that *collectively* describe the universe.

"Unexplained periodic fluctuations in the decay rates of Si-32 and Ra-226 have been reported by groups at Brookhaven National Laboratory (Si-32), and at the Physikalisch-Technische-Bundesandstalt in Germany (Ra-226). We show from an analysis of the raw data in these experiments that the observed fluctuations are strongly correlated in time, not only with each other, but also with the distance between the Earth and the Sun. Some implications of these results are also discussed, including the suggestion that discrepancies in published half-life determinations for these and other nuclides may be attributable in part to differences in solar activity during the course of the various experiments, or to seasonal variations in fundamental constants."

In physics, we examine Wolfram's proposal for a deterministic model underlying quantum mechanics, with 'long-range threads' to connect entangled particles. We show that this proposal cannot be made compatible with both special relativity and Bell inequality violation.

So Wolfram doesn't address actual questions, and when others do it for him, his ideas comes up empty of anything resembling actual physics.

[Hmm. It reminds me of something. Something with loops but (still?) without basic harmonic solutions, presumably incompatible with special relativity and QM, ... Oh!

Though it looks like a very interesting event that touches on many topics I am interested in and I will be in Europe around the time anyway, I won't be at the DICE 2008. (Well, it's so far mostly a passive interest). Best,

Indeed, during Stephen's talk somebody kept asking for entanglement, the reason for which I couldn't quite figure out since he was talking about GR, not about quantizing anything. Thanks for the links, I will give that a read. I find it kind of odd though that he expects people to buy his book, one would think if he was really interested in physics, he'd go publish his finds in a decent journal like people do in that field. Best,

It's a meeting of condensed matter and QG people, why would somebody give a talk about the Higgs at the LHC? I don't think I've heard the word Higgs the whole week, and as far as I can recall I've been the only one mentioning LHC. Theoretical physics doesn't equal collider physics. Best,

Dear Arun,Without having read the paper, I'd give it 98% chance the alleged discovery will vanish in a measurement glitch. Maybe they've used a random number generator that depends on the season, who knows. I guess the best thing is see and wait what happens. Besides this, I have of course an unpublished draft in my drawer that 'predicts' exactly this ;-) Best,

"In fact, as he admits, for Wolfram the real test of the complexity of a pattern is that it should look complex. Much of his discussion of complexity is anecdotal, relying on pictures of the patterns produced by specific automata that he has known. In this, Wolfram is allying himself with one side in the ancient struggle between what (with much oversimplification) one might call cultures of the image and cultures of the word.

[ Bee will like this, since she is against pop-culture promotion, videos, etc ]

In our own time it has surfaced in the competition between television and newspapers and between graphical user interfaces and command line interfaces in computer operating systems."

It seems as if SW has fallen into the trap of using Appearance ("pretty pictures"), rather than Substance ("functionality"). There was a critique by C. Adami/Caltech, where he pointed out SW's favorite case study..that of a sea shell. It had a visual pattern which looked exactly like a CA simulation ("picture"). The criticism is that SW is equating appearance (rather than functionality) to Coincidence/Correlation/Causality model. A fundamental mistake. Which explains C. Adami's strong reaction:

"Chris Adami, a Caltech theoretical physicist who is a leader in using computers to model complex living systems, dismissed Wolfram’s work as “pathetic” and “exasperating.”

It's the classic case of mind-intoxication in Research: "falling in love with your own Theory".

"Sometimes, the [ logical ] Brain has a [ subjective ] Mind of its own"-- anonymous

I think this is EXACTLY what is going on in String Theory. The RoI/Return on Investment model is showing cracks, that is what P. Woit/NEW & L. Smolin/"The Trouble with Physics" are saying: they've had 20 (?) odd years to come up with something & they haven't. The real damning thing is the *defensive* nature of the response by ST'ists, most notably Clifford Johnson. I mean, come on!! In an exchange with P. Woit, he REFUSES to read a criticism (a book or slide presentation, I forgot) & says "I know what they're talking about". AAARGH!! I get the CLEAR impression, that they know their "backs are against the wall" & they know ST is in trouble. Their only defense is just denial, lies, misleading statements, etc. A grown man (USC theoretical physicist, professor) behaving like a child! (denial, "got caught with hand in cookie jar"). I have had a personal incident with this guy (SUSY '06), where he was STALKING me with a camera, published a photo (without permission, you need a "model release") on Cosmicvariance. When I complained to S. Carroll/CV, he immediately removed the image, & decided to ban ME!? Later on, there was some a breakup between CV & C. Johnson & I think I know why. Later on, in his Asymptotia blog, he illegally trespasses in a park in Los Angeles (where he mentions acknowledging no-trespassing after dark sign, & flippantly infringes on it), & takes a picture of himself watching meteors!! Man, this guy is one mouse click from getting a visit from the police & arrested!

[ there are numerous cases of criminals taking pictures of themselves in the act..CVJ does it on his own blog!!?? WTF! ]

I mean, come on! Grounds for dismissal at USC, scandal for the ST community. Peter Woit's NEW campaign, just got the "smoking gun" it needs: a ST'ist has a pattern of illegal/criminal behavior, is arrested/jailed, loses professorship @USC.

I mean, did ANYBODY catch this infraction..it's right on his blog. It's called "paper trail", self-incrimination variety. I recall Kea saying on her blog "Clifford likes to chat, he's so sweet". (burying my head in discust). Later on, she sees CJ's true colors when CJ rips into poor Tommaso Dorigo (Lisa Randall CERN "incident"), prompting Kea to make a post "Huumph". She wasn't happy about CJ's erroneous criticism of TD. It goes back to the CV principals (S. Carroll & M. Trodden), who are trumpeting their outrage of sexism in Physics..when in fact S. Carroll is a hypocrite. Trumpeting his R. Feynman desk, when in fact RPF was a notorious sexist pig! (see Longnow article, where female employee is treated like a slave/housekeeper). The collective effect of this data-point, along with: interviewing grad student D. Goodstein (ex Caltech vice-provost) at a strip club in Pasadena, scoping for women on Colorado Bl, talking the female physics student to pose naked for him (he got in trouble with Physics Dept head for this). He's a great physicist, but a sexist (he was brought up in old school sexist environment).

I recall seeing a comment on CV by (apparently a female) poster:

"Sean & Mark stop just short of doing anything about sexism"

I.e., they're just doing lip-service ("talk the talk"), & now acting on solutions ("walk the walk"). That CV blog is just a TOOL for their political agenda: make themselves look like they're doing Physics outreach (when in fact they're disrespecting EVERY female physicist with their hypocrisy).

See, this illustrates the whole "intoxication" nature of research": pet theories, pet friends (you are blinded, can't see character flaws or worse..like CVJ). Basically, CV is a club & if you say anything against them (my complaint for being stalked by CVJ), you are banned! Sounds like High School bullsh*t.

"Life is like High School..WITH MONEY"-- David Letterman

So, getting back on topic. SW apparently got intoxicated with CA via "pretty pictures". And, made a major blunder ("Appearance over Substance"). The negative reviews ("preponderance of evidence") by his peers are sending SW a message: you blew it. SAME THING for String Theory, via P. Woit/NEW & L. Smolin/"The Trouble with Physics": ST is blowing it. The latter seem to have made serious inroads in damaging ST. Are they suggesting solutions (isn't L. Smolin into LQG?)? The problem is, I think, ALL of theoretical Physics is in trouble. ALL theories are like ST: it's gone WAY beyond ("outpaced") the data. I heard you have to build a particle accelerator the size of the galaxy to get data for any TOE.

I heard this from the Kea/Louise camp: "we are winning!". No, everyone is LOSING. If you don't have the data..NOBODY will get the right theory. Everyone is in the "Playing to Win" mode (way TOO optimistic), when in fact it's a losing scenario.

Unfortunately, for theoretical physics it seems to be "local pessimism, global pessimism" because of too much "Valor". There's the famous phrase:

"Discetion is the better part of Valor"

warning of trying to overstate your case.

Theory must follow the data, & the data simply isn't there (due to practicality constraints). An 80 million shortfall for Fiscal 2008 (?) doesn't help either (jeopardizing the next generation collider, the ILC).

Again a nice post, leaving one with much to wonder about and consider. The emergence of gravity, interesting and yet from Einstein’s perspective, where gravity is the observed interaction of space-time to matter/energy isn’t it whether these are to be considered emergent first truly the question? As an example in the brane clashing models is what’s clashing to be considered matter/energy or not? Likewise, is the distance between them to be considered space-time or not? For me it appears that when all is considered you still end up with simply a chicken and egg scenario. Perhaps we should call this self referencing circular physics. Such self referencing and circularity would register as a logical error even in the simplest of spread sheets and as such I find it hard to imagine how it considered to be appropriate to serve as the model for a TOE.

Oh yes as for Stephen Wollfram’s book it was something that was given to me sometime back which I never read from beginning to end, yet rather more or less jumped back and forth through. For the most part it doesn’t appear to offer anything all that new that hasn’t been addressed between Chaos/Complexity Theory and the idea that all is a computer. Actually my largest disappointment is that Wolfram appears to have a very shallow concept of what random is and what role emergence has in this regard. It never seems to occur to him that random itself may be the order of all orders where all the sequences and patterns are already there to be found and therefore no need for a cellular automaton action other then to serve as place marker. There really is nothing here that hasn’t been already suggested between people such as Cantor, Mandelbrot and Bohm, rather actually a lot less, well at least from my perspective that is.

In short Wolfram’s book rivals the length of Penrose’s ‘Road to Reality’ yet not its content as to what I would consider to be physics. Subjective perhaps and yet I would rather call it instinct or intuition, which of course are today on the black list in such considerations.

I went through the paper as best I could. The good thing about it is that if there is an anomaly as claimed, it should be relatively easy for it to be replicated, it will only take time.

How one measures directly the half-life of a radioactive nuclide with a half-life in the range of several years, is to directly count decays over a long period. To account for all kinds of possible errors with the apparatus, one also counts decays of another nuclide which has a much longer half-life and therefore which provides a constant count for the duration of the experiment. It is this ratio which shows the annual time dependency. The size of the effect is about 3 parts in a 1000. The data series are for 4 years and 15 years.

The authors say the data used is corrected for radon decay background - radon is known to have a seasonal variation.

The idea is roughly that concepts like the manifold that is our space-time and/or the metric on it are not fundamental ingredients of our theories but only emergent on a macroscopic level. I.e. fundamentally, there might be no such thing as a space-time, much like on the elementary level there is no such thing as fluid. Properties of liquids are 'emerging' if you look at them in a limited range of resolution. The idea is roughly that the smoothness of space-time is a similar effect, whereas on a deeper level completely different concepts are relevant (e.g. everything we know and like is a cellular automata or a graph, or something we don't know at all).

Can't publish in any journals but I will try to explain briefly for what it's worth.

They reject reductionism, and find no relevance for Navier Stokes in that process, so loose a lot of credibility by rejecting reductionism.

My earlier statement on Equilibrium recognizes this, and needs some proposal to cover this mistake by the Condense matter theorists, and recognizing an anomaly in the physic's extreme, become inclusive. Such a statement needed some phenomenological basis, and in reductionism, it takes you to this point?

You have to understand Susskind's 10-Sup-500-/sup. Smolin himself sees multi universe out of such a condition, yet never identified this condition in the blackhole.

Now this has changed, at least as I supply a new focus on an alternative emergent process in what unfolds in those valleys.

I am curious where do the "emergent gravitists" define the emergence? If you are at the low energy end of the scale then you are talking about the "Bohr correspondence". Nothing new here as it is defined as the "behaviour of quantum mechanical systems reproduce classical physics in the limit of large quantum numbers". There just does not happen to be much observable crosstalk between two such sytems i.e. GR and quantum theory. If someone tells you that Planck's constant goes to zero for classical systems that is not correct either as its value was fixed very close to the cosmological beginning and it remains fixed into the low-energy. Large systems produce new qualities or design which have effects that are not fundamental, that's all. An "emergent gravitist" could (possibly) talk about an emergence of spacetime or gravity from a pre spacetime only, and then it would be very improbable as there is no quantum large number to create non fundamental things at such an early time. I will agree that something like Newton's constant G is not fundamental but that is due to the semiclassical nature of gravity and its change of a large number of gravitons in the condensate and is not due to an emergence of a non-fundamental spacetime. Spacetime is created immediately after t = 0 and the 4d spacetime is an enlargement of a Hilbert space(s) from the cosmologic rolldown. This still leaves 6d or 22d involved which is still integrated with the 4d from the very beginning time.mark

Sorry, I don't understand the question. What do you mean with 'where' does one 'define' emergence? Do you mean with what parameter gravity would emerge? It's not that I'm very much into these emergent gravity things, but since it's supposed to be a quantum gravity effect, I'd think temperature resp. energy density resp. curvature is a good candidate. I don't know what this has to do with a limit hbar to zero or something of that sort. Best,

Bee, (Anonymous from jal)I'm very interested in knowing more about the Perfect Liquid (QGP) and how experiments at Tc, high pressure will match those from CERN.I've read a few papers from condensed matter that demonstrate their enthusiasm.What about "your" group?Did you find reasons to reject their models? jal

The truth of it is the concept of emergence is not a new one at all. Just as perhaps a point of interest one my favourite physicists/philosophers was in fact an emergence subscriber. That person of course was Rene Descartes. It is interesting to note that in his time he got himself into a bit of hot water for holding such views not only with theologians yet also with his fellow scientists.

“But this is certain, and an opinion commonly received among theologians, that the action by which he now sustains it is the same with that by which he originally created it; so that even although he had from the beginning given it no other form than that of chaos, provided only he had established certain laws of nature, and had lent it his concurrence to enable it to act as it is wont to do, it may be believed, without discredit to the miracle of creation, that, in this way alone, things purely material might, in course of time, have become such as we observe them at present; and their nature is much more easily conceived when they are beheld coming in this manner gradually into existence, than when they are only considered as produced at once in a finished and perfect state.”

-Rene Descartes- Discourse on The Method: of Rightly Conducting The Reason, and Seeking Truth in the Sciences (1637)

I never said it's a new concept. It is newer, I would think, to apply it so extremely as to erase the existence of space and time itself. I'm not good with the history of science, but I'd have thought the believe that somebody would find a fundamental theory that just describes everything was a very temporary trend that came along some decades ago, and seems to have vanished again. The idea to question that what we see and percieve, and what we've used in our descriptions, is really fundamental dates surely way back. Best,

Sorry I never meant it to be understood that the concept of emergence is something that you considered as being new, but rather perhaps by some of your readers. As it can be seen Descartes considered almost as those today that all could be at first simply Chaos to begin at that the laws (propensity of initial state) would be all that would account for it in the end. I would then think that all including gravity would be conceded by Descartes if asked. This might not have been the first time it was considered, yet certainly the first within the last millennium or so that I’m aware of.

In actuality my comment was meant also to demonstrate the depth of Descartes considerations, insight and conviction despite such views not just simply being unpopular yet potentially dangerous to hold. It find it good to cast the great thinkers not as just being wise yet also bold if not indeed brave which is something I feel is lacking in the public's general perception today.

Sorry about previous anon comment, as it naturally defaulted there by "hiding my ass." Type "" into google.

Atoms and Strings in the Laboratory?Well recall that one of the rather striking results from the plasma story was a measure of the hydrodynamic quantity the sheer viscosity of the fluid, usually presented as divided by the entropy

You had to identify the anomaly and creation in this gravitational collapse?

A simplified model would be in association to the spherical shell as a approximate, and you use that in cosmology as well?

My understanding is that the condense matter community seems to have experimental evidence that "the point of emergence" is when there is perfect symmetry which is in a perfect liquid where there is no mass, no gravity no CPT violation.jal

As a point of reference here because it hasn't been pointed out yet (and not because you don't all already know it) emergent gravity in the modern era is an idea from Andrei Sakharov in the late 60's. To get a fairly concise explanation of it in the modern form see the zero-point field article in wikipedia, which I've helped edit.

Re: lack of disagreement, emergence of spacetime and gravity are central topics in string theory, with fully working examples that avoid the well-known pitfalls. Those pitfalls (say, the Weinberg-Witten theorem, or lack of Lorentz invariance) seem to inflict lots of such attempts, old and new (and recycled), and seem to be just ignored for the most part...In view of that, it is strange no string theorists are in the list of participants, some experts were probably present at the same building during the conference.

The big problem with "emergence of gravity studies" is that people have a tendency to re-define "emergence" to mean whatever it is that comes out of their theories. String theorists in particular have this habit. AdS/CFT is a great thing, but it does not cure all known diseases; certainly not this one.

It is not a point of redefining as Tom Bull suggests. It is recognizing the limitation with which reductionism has run it's course, unless one seeks greater energies and a larger playing field in which to investigate. Cosmologically this process of reductionism is being emulated in the playing field of space and one has only to apply this dual nature of looking at cosmic particle and developing them here in the LHC for experimental purposes to understand the univeses evolution?

Now who is it that reiterates that it does not matter which building blocks you use?

If one understands that the process in reductionism is being moved back to the microseconds then one should understand that this emergent system is being directed "exactly" to where that emergent gravity arises.

Witten himself, pointed out this "emergent process" and condoned the condense matter theorist point of view? Pierre Ramond and others of course keep pointing this out.

Sorry, if I intrude on those who at what school were around, but it does not take a physicist or theoretician to see the necessity of the picture I just showed.

Back to Stephen Wolfram. Lawrence Gray has an interesting, informative, and partially critical, partially laudatory review in the AMS Notices, "A Mathematician Looks at Wolfram’s New Kind of Science," Notices of The American Mathematical Society, vol. 50, no. 2, (2003), pp. 200--211. A PDF is freely available on-line here.

There is a lot of stuff at the beginning about universal Turing machines. To orient yourself, think, "The undecidability of the halting problem and its consequences."

Gray's list of recommended readings on page 202 is extensive and particularly interesting, especially "Book review by Scott Aaronson for Quantum Information and Computing, September 2002. (Proves that Wolfram’s proposed discrete model for the universe cannot accommodate both special relativity and Bell’s inequality violations.)". (I know more recent versions of Scott Aaronson's critique have been mentioned by previous commentators.)

Gray also includes a telling discussion of "intrinsic randomness generation" on page 209.